Normative: Align time zone IDs across engines

This PR resolves #825 by adding spec text that more clearly defines
how ECMA-402 implementations should decide which IANA time zone IDs
should be primary vs. non-primary.

This PR implements "Option C" in #825 by deterministically defining
ECMAScript's exceptions from the IANA Time Zone Database's defaults,
and then pointing implementers at ICU as a convenient implementation
of those exceptions.

This PR also accommodates to web reality by aligning the 402 spec text
with the existing behavior of ICU and CLDR while providing deterministic
rules that can guide future changes in CLDR data.

Finally, this PR introduces a new StringSplitToList abstract operation
and uses it to simplify AvailableNamedTimeZoneIdentifiers and
IsWellFormedUnitIdentifier.